Why choose PNG as the output
PNG is lossless, so a converted picture can be opened, edited and saved again without further quality loss. It also supports transparency, which is why designers and developers ask for it so often.
The cost is file size. A photograph stored as PNG is usually several times larger than the same picture as JPG or WebP, so use it where fidelity matters more than weight.
What conversion cannot do
Changing the format never restores detail that an earlier lossy format discarded. A PNG made from a JPG is an exact copy of that JPG's pixels, not an improvement on them.